Not Applicable
The present invention relates to mixed-resistance structured packing and methods for assembling such packing in an exchange column. The mixed-resistance structured packing has particular application in exchange columns, especially in cryogenic air separation processes, although it also may be used in other heat and/or mass transfer processes that can utilize structured packing.
The term, xe2x80x9ccolumnxe2x80x9d, as used herein, means a distillation or fractionation column or zone, ie., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term xe2x80x9ccolumn sectionxe2x80x9d (or xe2x80x9csectionxe2x80x9d) means a zone in a column filling the column diameter. The top or bottom of a particular section or zone ends at the liquid and vapor distributors, respectively.
The term xe2x80x9cpackingxe2x80x9d means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are xe2x80x9crandomxe2x80x9d and xe2x80x9cstructuredxe2x80x9d.
xe2x80x9cRandom packingxe2x80x9d means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
xe2x80x9cStructured packingxe2x80x9d means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
There are many processes for the separation of air by cryogenic distillation into its components (i.e., nitrogen, oxygen, argon, etc.). A typical cryogenic air separation unit 10 is shown schematically in FIG. 1. High pressure feed air 1 is fed into the base of a high pressure column 2. Within the high pressure column, the air is separated into nitrogen-enriched vapor and oxygen-enriched liquid. The oxygen-enriched liquid 3 is fed from the high pressure column 2 into a low pressure column 4. Nitrogen-enriched vapor 5 is passed into a condenser 6 where it is condensed against boiling oxygen which provides reboil to the low pressure column. The nitrogen-enriched liquid 7 is partly tapped 8 and is partly fed 9 into the low pressure column as liquid reflux. In the low pressure column, the feeds (3,9) are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components. Structured packing 11 may be used to bring into contact the liquid and gaseous phases of the oxygen and nitrogen to be separated. The nitrogen-rich component is removed as a vapor 12, and the oxygen-rich component is removed as a vapor 13. Alternatively, the oxygen-rich component can be removed from a location in the sump surrounding reboiler/condenser 6 as a liquid. A waste stream 14 also is removed from the low pressure column. The low pressure column can be divided into multiple sections. Three such sections with packing 11 are shown in FIG. 1 by way of example.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils (or corrugated mesh cloths) stacked vertically. These foils may have various forms of apertures and/or surface roughening features aimed at improving the heat and mass transfer efficiency. An example of this type of packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
The separation performance of structured packing is often given in terms of height equivalent to a theoretical plate (HETP). The term xe2x80x9cHETPxe2x80x9d means the height of packing over which a composition change is achieved which is equivalent to the composition change achieved by a theoretical plate. The term xe2x80x9ctheoretical platexe2x80x9d means a contact process between vapor and liquid such that the existing vapor and liquid streams are in equilibrium. The smaller the HETP of a particular packing for a specific separation, the more efficient the packing because the height of packing being utilized decreases with the HETP.
The efficiency of distillation columns with structured packing shows a dependency on their diameter when all the other geometric and process factors are held constant. While performing equivalent separations at different scales, as the diameter increases from a small fraction of a meter to several meters, the HETP increases first and then tends to level out. This may be explained by a combination of two factorsxe2x80x94the flow characteristics and the mixing characteristics of structured packing columns.
In terms of flow characteristics, even when the initial liquid and vapor distribution into a packed section of a column is highly uniform, the distribution changes as the liquid and vapor flow in countercurrent contact through the packed section, resulting in variations in the liquid to vapor (L/V) ratio across the cross section of the column. Also, it is known that a significant flow of liquid occurs at the column wall, thereby reducing the liquid loading in the packing in an annular region of the packing adjacent the wall. The vapor flow, although not completely uniform, is more uniform within the packing than is the liquid flow.
Thus, there usually is a systematic variation in the L/V ratio across the cross section of a typical cylindrical packed column as shown schematically in FIG. 2. Referring to FIG. 2, in a typical cylindrical packed column 22, there is an annular space 19 between the column inner wall 40 and the packing, which is disposed between the parallel broken lines 16 (representing the perimeter of a cylindrical layer of packing). The column axis is represented by broken line 15. Broken line 17 represents the xe2x80x9cnominalxe2x80x9d L/V ratio for theoretical or ideal conditions where there would be no variation in the L/V ratio across the cross section of the column. Solid line 18 is a schematic representation of the non-uniform L/V ratio (relative to nominal) across the cross section of a typical cylindrical packed column. The L/V ratio is much higher near the column inner wall because of excessive liquid flowing down the column inner wall (as indicated by the steep slope of line 18 above annular space 19 in FIG. 2).
The general pattern of the actual L/V ratio illustrated by line 18 in FIG. 2 may vary considerably depending on the details of the packing, the mixture being separated, and the process conditions.
Further, it is well known that maldistribution can result in degradation of the separation efficiency of the column unless it is mitigated by repeated mixing of the liquid and vapor phases within the column. This is especially true for tight separations such as those used in cryogenic air separation.
In terms of mixing characteristics, a small diameter column with a large length to diameter (l/d) ratio (e.g., about 5 to 20) can mix the vapor flow and, to a lesser extent, the countercurrent liquid flow repeatedly across the column cross section, which can average out the consequences of local variations in L/V ratios much better than a large diameter column with a much lower l/d ratio (e.g., about 0.5 to 5.0). For this reason, the degradation in separation efficiency compared to the ideal is more severe in large diameter columns, which results in an increase in HETP.
The increase in HETP in large exchange columns is a major economic penalty, since it increases the overall height of the system of which the column is a part. It is desired to mitigate the increase in HETP in large diameter columns, so that such columns may approach the performance of small diameter columns in terms of separation efficiency.
The prior art has not recognized or addressed this specific problem. The prior art has recognized the deleterious effect of excessive wall liquid flow, and there have been attempts to mitigate that effect, such as by the use of conventional wall wipers. However, although wall wipers can reduce wall liquid flow locally, wall wipers are not very effective in returning liquid back to the packing. Thus, even in columns equipped with wall wipers, there still are unfavorable variations in L/V ratios. The deleterious effect of vapor bypass at the column wall can be mitigated by the use of restricting means in the annular space near the column wall, such as the solid metal wipers and other devices disclosed in the following copending U.S. patent application assigned to the assignee of the instant application: Ser. No. 09/166373 to Klotz, et al. entitled xe2x80x9cDevices to Minimize Vapor Bypass in Packed Column and Method of Assemblyxe2x80x9d, now abandoned.
U.S. Pat. No. 5,262,095 (Bosquain et al.) describes the use of packing edge modification by deformation, slits, porous plugs, fillers or special wipers in order to promote a flow reversal of liquid back into the packing and away from the wall of the column. U.S. Pat. No. 5,441,793 (Suess) describes the use of liquid re-director elements at the packing edges near the wall. Such elements may be made out of xe2x80x9cLxe2x80x9d shaped mini corrugations. U.S. Pat. No. 5,224,351 (Jeannot et al.) describes similar edge modifications by folding some of the corrugation edges near the column wall. U.S. Pat. No. 5,700,403 (Billingham et al.) describes the formation of special corrugated packing layers wherein alternate corrugated elements within a structured packing layer near the wall are cut short so that the tendency to lead liquid towards the wall is reduced. U.S. Pat. No. 5,282,365 (Victor et al.) describes the use of heat addition at the column wall in order to vaporize and reduce wall flow.
While the packings and methods taught in the first four patents may reduce wall liquid flow, the associated costs are expensive, since the manufacturing techniques are unconventional and installation of the packings would likely be labor intensive. The proposed solution of the fifth patent also would be expensive, because it would require additional process circuitry to bring another fluid outside the column in order to evaporate the wall liquid inside the distillation equipment.
U.S. Pat. No. 5,100,448 (Lockett et al.) discloses the use of structured packing of different packing density in at least two sections of a column which are directly above and below each other to balance hydraulic loading. Likewise, in U.S. Pat. No. 5,419,136 (McKeigue) the corrugation angle of the structured packing is varied in two sections which are directly above and below each other for the purpose of balancing hydraulic loading. Although these arrangements of packing reportedly provide improved operating flexibility in cryogenic air separation, they do not address the problems of maldistribution discussed herein, nor do they provide a solution or a suggestion of a solution for any of those problems.
It is desired to have a structured packing which minimizes the effects of maldistribution using a variation of conventional structured packing which does not require any special edge modification of the packing or any additional equipment or circuitry outside the exchange column.
It is further desired to have a structured packing that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications. Specifically, it is desired to mitigate the increase in HETP in large diameter columns used in such applications, so that such columns approach the performance of small diameter columns in terms of separation efficiency.
It is still further desired to have an exchange column wherein the overall liquid to vapor (L/V) ratio within the column deviates as little as possible from the nominal (excluding wall effects), thereby resulting in an improved mass transfer performance.
It is still further desired to have an exchange column having a structured packing wherein the L/V ratio is maintained nearly constant in the column even if the absolute liquid and vapor flows are not maintained constant.
It is still further desired to balance the L/V ratio across the cross section of an exchange column and to make large diameter columns approach the performance of small diameter columns in mass and/or heat transfer efficiency.
The present invention is a layer of mixed-resistance structured packing, which may be used in one or more sections of an exchange column for exchanging heat and/or mass between a first phase and a second phase in a process, such as cryogenic air separation. The invention also provides a method for assembling such a layer of mixed-resistance structured packing in an exchange column. Other aspects of the invention are a method and a system for reducing HETP (height equivalent to a theoretical plate) in exchange columns.
The mixed-resistance structured packing may be used in one or more layers of packing in one or more sections of an exchange column. In such a layer of mixed-resistance structured packing, lower resistance packing is used in the central core and a higher resistance packing is used in an outer annulus surrounding the central core. This forces more vapor flow toward the center of the exchange column and less toward the column wall, thereby counteracting a tendency of liquid to maldistribute in the exchange column. By using the method of the present invention to balance the L/V ratio (liquid to vapor ratio), large diameter columns approach the performance of small diameter columns in terms of substantially lower HETP.
In one embodiment, the layer of mixed-resistance structured packing comprises: a first structured packing having a first packing resistance; and a second structured packing generally horizontally adjacent the first structured packing, the second structured packing having a second packing resistance different than the first packing resistance.
In one variation, the second structured packing has an inner perimeter substantially equal to the outer perimeter of the first structured packing and an outer perimeter greater than the inner perimeter. The inner perimeter of the second structured packing substantially abuts the outer perimeter of the first structured packing. In another variation, the outer perimeter of the first structured packing and the inner perimeter of the second structured packing are substantially circular.
In another variation, the first and second structured packings comprise at least one corrugated plate. In yet another variation, the first and second structured packings comprise a plurality of corrugated plates made of foil-like material disposed in parallel relation, each said plate having at least one corrugation disposed at an angle and in a crisscrossing relation to at least one corrugation of an adjacent plate. A difference in resistance between the first and second structured packings may be due to a difference in the angles of the corrugations. For example, the angle of the at least one corrugation in the first structured packing may be different than the angle of the at least one corrugation in the second structured packing.
In yet another variation, a difference in resistance between the first and second structured packings is due to a difference in surface area density of the first and second structured packings. For example, the surface area density of the second structured packing may exceed the surface area density of the first structured packing.
Another embodiment of the invention is a layer of mixed-resistance structured packing comprising: a substantially circular central core having an outer perimeter, the central core comprising a first structured packing having a first packing resistance; and an outer annulus generally horizontally adjacent the outer perimeter of the outer core, the outer annulus comprising a second structured packing having a second packing resistance different than the first packing resistance.
Another aspect of the present invention is an exchange column for exchanging heat and/or mass between a first phase and a second phase, the exchange column having at least one layer of mixed-resistance structured packing as in any one of the embodiments or variations described above.
Yet another aspect of the present invention is a process for cryogenic air separation comprising contacting vapor and liquid counter-currently in at least one distillation column containing at least one mass transfer zone wherein liquid-vapor contact is established by at least one layer of mixed-resistance structured packing as in any of the embodiments and variations described above.
The present invention also includes a method for assembling a layer of mixed-resistance structured packing in an exchange column comprising multiple steps. The first step is to provide an exchange column. The second step is to provide a layer of mixed-resistance structured packing, the layer of mixed-resistance structured packing comprising: a first structured packing having a first packing resistance; and a second structured packing generally horizontally adjacent the first structured packing, the second structured packing having a second packing resistance different from the first packing resistance. The final step is to install the layer of mixed-resistance structured packing in the exchange column.
Another aspect of the present invention is a method for reducing HETP (height equivalent to a theoretical plate) in an exchange column for exchanging heat and/or mass between a liquid and a vapor, the exchange column having at least one layer of structured packing, the layer of structured packing having a central core and an outer annulus generally horizontally adjacent the central core. The method comprises the following steps: inducing at least a portion of the vapor in the exchange column away from the outer annulus; and inducing the at least a portion of the vapor toward the central core. In one variation of the method for reducing HETP, the portion of the vapor is an amount whereby the liquid-vapor ratio across a cross section of the exchange column is maintained at nearly a constant value.
Yet another aspect of the invention is a system for reducing HETP in an exchange column for exchanging heat and/or mass between a liquid and a vapor, the exchange column having at least one layer of structured packing, the layer of structured packing having a central core and an outer annulus generally horizontally adjacent the central core. The system comprises: means for inducing at least a portion of a vapor in the exchange column away from the outer annulus; and means for inducing the at least portion of the vapor toward the central core. In one variation of the system, the portion of the vapor is an amount whereby the liquid-vapor ratio across a cross section of the exchange column is maintained at nearly a constant value.
Another aspect of the present invention is a packed section in an exchange column comprising: a first layer of mixed-resistance structured packing (as in any one of the embodiments or variations described above); and a second layer of mixed-resistance structured packing (as in any one of the embodiments or variations described above) located below the first layer of mixed-resistance structured packing, wherein the second layer is rotated at an angle relative to the first layer. The angle may be between about 0xc2x0 and about 90xc2x0.